1. Field of the Invention
The present invention relates to elastic wave devices having unbalanced-balanced conversion functions, and particularly relates to elastic wave devices having five IDT-type unbalanced-balanced conversion functions and duplexing devices that use such elastic wave devices.
2. Description of the Related Art
In recent years, duplexers are being connected to antenna terminals in mobile wireless communication applications, such as in cellular phones. A duplexer includes a transmission filter and a reception filter. An unbalanced-balanced filter circuit is used in the reception filter in order to connect to an IC in the next stage.
Japanese Unexamined Patent Application Publication No. 2001-313540 discloses an elastic wave device for configuring this type of unbalanced-balanced filter circuit. FIG. 10 schematically illustrates the circuit configuration of the elastic wave device.
As shown in FIG. 10, an elastic wave device 1001 includes an unbalanced signal terminal 1002, a first balanced signal terminal 1003, and a second balanced signal terminal 1004. The elastic wave device 1001 is a five IDT-type longitudinally coupled resonator-type surface acoustic wave filter. As shown in FIG. 10, first to fifth interdigital transducers (IDTs) 1011 to 1015 are disposed along an elastic wave propagation direction. The first to fifth interdigital transducers 1011 to 1015 are configured so that the phase of a signal obtained at the first balanced signal terminal 1003 and the phase of a signal obtained at the second balanced signal terminal 1004 are opposite from each other.
Meanwhile, Japanese Unexamined Patent Application Publication No. 2003-249842, indicated below, discloses a duplexer that employs an unbalanced-balanced conversion elastic wave filter such as that mentioned above as a reception filter circuit.
FIG. 11 is a block diagram schematically illustrating the duplexer according to Japanese Unexamined Patent Application Publication No. 2003-249842. A duplexer 1021 includes an unbalanced signal terminal 1002 connected to an antenna terminal 1022. A transmission filter 1023 is connected between the unbalanced signal terminal 1002 and a transmission terminal 1025. A reception filter 1024 is connected between the unbalanced signal terminal 1002 and first and second balanced signal terminals 1003 and 1004. Note that a matching circuit 1026 is connected between the reception filter 1024 and the unbalanced signal terminal 1002.
The elastic wave device 1001 has an unbalanced-balanced conversion function such as that described above. However, there has been a problem in that when the elastic wave device 1001 is used, differential transmission characteristics, including isolation characteristics, between transmitted signals and received signals have been insufficient. This will be described with reference to FIGS. 12 and 13.
In FIG. 12, the solid line indicates amplitude characteristics between the unbalanced signal terminal 1002 and the second balanced signal terminal 1004 in the elastic wave device 1001, whereas the broken line indicates amplitude characteristics between the unbalanced signal terminal 1002 and the first balanced signal terminal 1003. As is clear from FIG. 12, the amplitude characteristics differ between the two. The reason for this is as follows. An electrical signal obtained from the first balanced signal terminal 1003 and an electrical signal obtained from the second balanced signal terminal 1004 have opposite phases. However, signals transmitted in a bridged manner between the unbalanced signal terminal 1002 and the first and second balanced signal terminals 1003 and 1004 have the same phase. Accordingly, in the case where the signals transmitted in a bridged manner have the same phase as the electrical signal flowing from the unbalanced signal terminal 1002 to the second balanced signal terminal 1004, for example, the signals will strengthen each other. In this case, the signal flowing from the unbalanced signal terminal 1002 to the first balanced signal terminal 1003 has a phase opposite from the stated signals transmitted in a bridged manner, and thus these signals will weaken each other. A difference in amplitude characteristics between the signal obtained from the first balanced signal terminal 1003 and the signal obtained from the second balanced signal terminal 1004 will arise as a result, as shown in FIG. 12. This difference is more marked in higher frequencies.
Assume now that the duplexer shown in FIG. 11 is configured using the elastic wave device 1001 as a reception filter circuit. In this case, the amplitude characteristics of the signal transmitted from the transmission terminal 1025 to the first balanced signal terminal 1003 or the signal transmitted from the transmission terminal 1025 to the second balanced signal terminal 1004 will be as indicated by the solid line and the broken line shown in FIG. 13. Accordingly, transmission characteristics, isolation characteristics, and so on tend to drop when operating in a differential mode.